Wednesday, November 6, 2013
Environmental Science and Engineering Seminar
The Diversity and Evolution of Aerobic Respiration and Denitrification
James Hemp, Agouron Postdoctoral Scholar, Geological & Planetary Sciences, Caltech
High redox potential metabolisms play a central role in both modern biogeochemical cycles and the coevolution of life and Earth. The electron transfer chains of these metabolisms share a common core architecture composed of a small number of evolutionarily related protein families. The recent explosion in genomics has led to the discovery of extensive diversity within these families, however detailed biochemical knowledge of these proteins are required to determine their evolutionary histories.
The heme-copper oxidoreductase (HCO) superfamily catalyzes crucial reactions in both aerobic respiration and the nitrogen cycle. We used comparative genomics, coupled with structural and biochemical analyses, to characterize the diversity of the HCO superfamily. A number of previously unidentified families were discovered, many of which catalyze nitric oxide reduction. This greatly expands the known diversity of organisms capable of performing denitrification, suggesting that this metabolism is more widespread than previously recognized. We also identified a putative nitric oxide dismutase, an enzyme capable of O2 production, which has significant geobiological implications. Phylogenomic analyses suggest that aerobic respiration originated within the Cyanobacteria after the evolution of oxygenic photosynthesis, and was later distributed to other clades via lateral gene transfer. Finally we demonstrate that nitric oxide reductase activity evolved many times independently from various oxygen reductase members of the HCO superfamily, suggesting that canonical denitrification evolved after aerobic respiration.